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T O P I C R E V I E W

Robert Pearlman

International Astronomical Union release:

quote:The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are listed on IAU's "dwarf planet" watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

The "dwarf planet" Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects.

Below are the planet definition Resolutions that were passed.

RESOLUTIONS

Resolution 5A is the principal definition for the IAU usage of "planet" and related terms.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects.

IAU Resolution: Definition of a Planet in the Solar System

Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

RESOLUTION 5A

The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

1. A planet* is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

2. A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape**, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

3. All other objects*** orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

** An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

*** These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies. The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

RESOLUTION 6A

The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

[Edited by Robert Pearlman (August 24, 2006).]

Rob Joyner

Alan Stern, leader of the New Horizon Pluto mission and Southwest Research Institute scientist, calls it "absurd" that only 424 out of some 10,000 professional astronomers were allowed to vote. That's only 5%!And regarding one of the new criteria that a body is considered a planet when it "has cleared the neighborhood around its orbit", Stern points out that Mars, Jupiter, Neptune and even Earth all have asteroids as neighbors."It's patently clear that Earth's zone is not cleared," Stern says, "Jupiter has 50,000 trojan asteroids." Hmm. Sound's kinda funny calling Jupiter a 'dwarf planet', huh? But it fits the definition.And "All other objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies"? Sound's like the 'scientific' equivalent to 'Space Thingy' to me...I wouldn't start tearing out the Pluto pages of any books just yet. This ain't over!

Rick Boos

I agree with you Rob!

mjanovec

To me, it seems like a giant waste of time to start re-writing to definitions on what we call things, just for the sake of satisfying some astronomers who place tremendous value on such things. Pluto has been considered a planet for ~70 years. Why change it now? If we change it today, who is to say we won't change it again in four years, then again a few years after that.

I would like to see that any orbiting body around the sun that meets a certain threshold for size and/or mass, and that isn't the satellite of another planet, should be called a planet. That size/mass threshold should be the mass of Pluto, since Pluto has been called a planet for ~70 years. Seems like a good cutoff point to me. If that means adding a 10th or 11th planet to the solar system list, so be it.

Robert Pearlman

Moments ago, I completed an interview with NPR to air tomorrow morning about the commercial effects of Pluto being demoted. We spoke about the text book market, the toy and model market, astronomy software, websites and past cultural references such as in sci-fi shows and movies. I also touched on New Horizons and its "lost" mission, the precedence of decisions such as these and my regret for the effect this will have on Clyde Tombaugh's legacy...

FFrench

I've noticed a lot of media reports titled "Pluto no longer a planet" or similar. To my mind, this is rather misleading - Pluto has been recategorized to something called a "dwarf planet." Still a name with "planet" in it... which I think that, considering the major schism here has been of an historical / emotional nature (in which I share) rather than a hard-science one, is a much more nuanced compromise than "not a planet". Of course, that doesn't make for as big of a media story...

Shuttlefan

Farewell Pluto, the family of ´real planets´ kicks you out. You´ve always been the black sheep with your eliptic orbit ( a little eliptic is smart but a real nice planet doesn´t overact) and your strange inclination. Now you get the bill! And regrets to you, Mr Tombaugh , wherever you may be, you didn´t find a planet, it was just a dwarf. We must be quite careful not to confuse ourselves in astronomy, white dwarfs, red dwarfs, brown dwarfs and now dwarfs which aren´t even stars and have no colour... Science is knowledge a n d history and you can preserve history and integrate new knowledge as well. You´re right, that´s not the final curtain, and so far my solar system books (got lots of them) can stay on the shelves.

Robert Pearlman

quote:Originally posted by FFrench:...is a much more nuanced compromise than "not a planet". Of course, that doesn't make for as big of a media story

True, but its partially justified by the distinction. From SPACE.com's story:

quote:Dwarf planets are not planets under the definition, however.

"There will be hundreds of dwarf planets," Brown predicted. He has already found dozens that fit the category.

Thusly, being one of 100 rather than being one of eight is significantly less special.

Blackarrow

I suggest that the opponents of Pluto's status as a proper planet have mounted a very clever, very subtle con-trick to win the argument. Instead of coming right out with a proposal to strip Pluto of its status (which would probably have been defeated), they put forward last week's absurd proposition that Pluto should remain a planet, and that "Xena", Ceres and Charon should also be classified as planets, with others eventually joining an increasingly unwieldy list. The idea that Ceres and Charon could be treated as planets beggared belief, and perhaps that was the plan all along. So absurd was last week's plan that scrapping it in favour of today's relatively simple plan must have attracted support which would not have been available last week. The people who got rid of Pluto are wasting their talents in the world of astronomy. They could go far as politicians. Damn them for what they have done....but I admit to a feeling of grudging admiration for their tactics.

dtemple

Those who voted for the change may (or may not) look a bit foolish after New Horizons flies past Pluto. I think this vote should have waited for the results of the Pluto flyby before making a decision. Typically, scientists are very surprised by the data and photos they receive from interplanetary spacecraft. There is a chance their decision will need to be reversed in the future.

Text books aside, there are hundreds if not thousands of products that were or are now on the market that include Pluto as the ninth planet of the solar system.

On Thursday, 424 astronomers voted to strip the small body of its planethood. Instead, Pluto is now categorized a "dwarf planet," one of hundreds that might be identified in the coming months and years.

In essence, the International Astronomical Union decided that when it came to planets, eight really was enough.

This may be bad news for Pluto fans, but for collectors, it presents an opportunity.

I think the fact that they used the excuse that Pluto is not a planet because it crosses the orbit of Neptune is ridiculous. We can come up with whatever definition we choose of what constitutes a planet, but if nothing else Pluto deserves planethood based on tradition for the past 70 years. I think Pluto should be "grandfathered in" as a planet.

KSCartist

The only "dwarfs" are the minority of professional astronomers who could make th trip for the conference and whose goal was to revise history.

No space geek is going to tell this space geek what to call Pluto. Pluto will always be a planet to me (and of course Mickey's dog).

Tim

mensax

By calling Pluto a dwarf planet... does that mean it is no longer a planet?

Is a dwarf person no longer a person? Is it politicaly correct to call a person a dwarf? Wouldn't it be better to call them "hieghth challenged"? Or better yet, might it not be more polite to not point out others shortcomings? (pun intended)

I wonder how we would feel if future discovered life forms, in Jupiters upper atmosphere, referred to the inner planets a dwarf planets... declaring to the universe that this was a four planet system.

Noah

Robert Pearlman

quote:Originally posted by mensax:By calling Pluto a dwarf planet... does that mean it is no longer a planet?

All jokes aside, yes. Per the IAU definition, dwarf planets are not planets, despite the similarity in names.

Robert Pearlman

From the National Air and Space Museum:

quote:The Exploring the Planets gallery will need the most work, not the least of which might be changing the lyrics to The Family of the Sun. Sung by a group of schoolchildren, the song has been playing continuously in the gallery since it opened in 1979.

Museum employees paid tongue-in-cheek tribute to the former planet by draping black crepe paper over the Pluto exhibit label. In addition, they placed the international symbol for "no," a red circle with line through it, over the Pluto sign at the gallery's entrance.